Iv'e found...over 6 decades...it normally comes from the sky under the label "Rain"...or Scottish airborne Spring water.Try rubbing silicon grease into the door seals to swell them up and check for perishing/fit. You will find two rear and two front openings under the sills. Poke summat in them like a wire coat hanger and clear them.You'll likely have rust in there...just to cheer you up!You don't want water near the passenger footwell by the way as it may travel to the ECU buried in there. If it gets damp or wet...your in trubs.(Big Trubs)

There is always an imbecile you never bargained for. Usually in a Tescon carpark with a dented Zafira minus hubcaps